Isn't one of FE tenets that the heavens are a dome?
Nope. I welcome you to try and find something about a dome in Earth Not a Globe or any of the other Flat Earth literature sources.
No matter, for all of us, FE or Spherical, the sky appears to be a semi-sphere.
The sky is not a sphere. How would things curve against it if it were not a literal sphere? Straight lines would be straight, unless they were resting against something curved.
Now that I comprehend that there are different beliefs and that you do not believe in a dome, I will not use that again with you, but I have seen FE believers say it is so, and have seen one analogy of a bowl with reflections to describe how both the north and south pole rotations can 'work'... but it made thing no clearer to me. That said, please allow me to try a different tack to the same attempt at communication. Have you ever gone to a planetarium? If so, you have seen that the sky is represented as a dome. That does not mean that we believe that the stars are on the same plane, but it is a great way to visually and accurately represent what is happening in the heavens.
So the sky is not a dome, but even as perspective on a two-dimensional piece of paper can accurately represent how our eyes see the world, so does a planetarium's dome accurately represent how our eyes see the stars and their motion as they appear to traverse the heavens.
Perspective of straight lines on a dome will look straight at one spot in the domed room but curved at another spot in the room. This, as I have previously stated, is due to perspective. Yet even to the person who sees the line as straight, the line drawn on the dome is in fact a curve. Only a string from one point on the dome to another point on the dome will prove what is the true straight vector from point a to point b. Thus, if point a is a radiant source such as the sun, perspective will obfuscate the true phase angle of a large body (point b, or the moon) reflecting the radiant energy from point a (our sun).
Video or pictures taken will reflect that poor perspective is the camera is not held to the proper orientation, or 2 dimensional plane of the camera's horizon. For instance, if the camera is held at a 45 degree angle to the earth's horizon, the camera reflects the horizon as sloping the opposite direction than the camera is being held. Also, if a close object such as wall-ceiling interface with straight lines is above or below the camera horizon, even though the camera is oriented to the straight, horizontal line, there will be an apparent curve at the foreground to that horizontal line as these lines move away to the right and left toward the convergence point of perspective. A similar horizontal line that is centered on the camera's horizontal plane with have no slope to the right and left. Some have called this lens distortion when what it truly shows is perspective. Lens distortion will simply magnify this effect based upon whether it is a wide angle, or has some magnification.
Therefore, straight lines will appear curved whether on the ground or in the sky based upon our vision being oriented to a different plane of reference than the one which is being observed. In order to correct for perspective in a camera or with regard to our own vision. we must put our camera's horizon which starts at center point on the left and is a straight line to center point on the right side of the field of view. If the camera is oriented with the moon at center point on the left or right with the phase angle exactly 90 degrees to the horizon of the camera, then when we find the sun at the opposite side of the field of view it will also be centered between the top and bottom of the camera viewfinder. Now by keeping the sun and moon both on the right and left, but moving the camera up or down in relation to this plane described by the sun and moon, you will see the phase angle of the moon begin to shift due to perspective as the two heavenly bodies begin to rise above or below the center-line of the camera's viewfinder as the plane that the sun and moon are on rises above or falls below the observed plane, even as the line of the wall/ceiling interface 'curves' when the field of view is below the plane of the ceiling.